This invention is generally in the field of Nuclear Magnetic Resonance (NMR) based techniques, and relates to a device and method for magnetic resonance imaging (MRI). Although not limited thereto, the invention is particularly useful for medical purposes, to acquire images of cavities in the human body, but may also be used in any industrial application.
MRI is a known imaging technique, used especially in cases where soft tissues are to be differentiated. Alternative techniques, such as ultrasound or X-ray based techniques, which mostly utilize spatial variations in material density, have inherently limited capabilities in differentiating soft tissues.
NMR is a term used to describe the physical phenomenon in which nuclei, when placed in a static magnetic field, respond to a superimposed alternating (RF) magnetic field. It is known that when the RF magnetic field has a component directed perpendicular to the static magnetic field, and when this component oscillates at a frequency known as the resonance frequency of the nuclei, then the nuclei can be excited by the RF magnetic field. This excitation is manifested in the temporal behavior of nuclear magnetization following the excitation phase, which in turn can be detected by a reception coil and termed the NMR signal. A key element in the utilization of NMR for imaging purposes is that the resonance frequency, known as the Larmor frequency, has a linear dependence on the intensity of the static magnetic field in which the nuclei reside. By applying a static magnetic field of which the intensity is spatially dependent, it is possible to differentiate signals received from nuclei residing in different magnetic field intensities, and therefore in different spatial locations. The techniques which utilize NMR phenomena for obtaining spatial distribution images of nuclei and nuclear characteristics are termed MRI.
In conventional MRI techniques, spatial resolution is achieved by superimposing a stationary magnetic field gradient on a static homogeneous magnetic field. By using a series of excitations and signal receptions under various gradient orientations, a complete image of nuclear distribution can be obtained. Furthermore, it is a unique quality of MRI that the spatial distribution of chemical and physical characteristics of materials, such as biological tissue, can be enhanced and contrasted in many different manners by varying the excitation scheme, known as the MRI sequence, and by using an appropriate processing method.
The commercial application of MRI techniques suffers from the following two basic drawbacks: the expenses involved with purchasing and operating an MRI setup; and the relatively low signal sensitivity which requires long image acquisition time. Both of these drawbacks are linked to the requirement in standard MRI techniques to image relatively large volumes, such as the human body. This necessitates producing a highly homogeneous magnetic field over the entire imaged volume, thereby requiring extensive equipment. Additionally, the unavoidable distance between a signal receiving coil and most of the imaging volume significantly reduces imaging sensitivity.
There are a number of applications in which there is a need for imaging relatively small volumes, where some of the above-noted shortcomings may be overcome. One such application is geophysical well logging, where the xe2x80x9cwhole bodyxe2x80x9d MRI approach is obviously impossible. Here, a hole is drilled in the earth""s crust, and measuring equipment is inserted thereinto for local imaging of the surrounding medium at different depths.
Several methods and apparatuses have been developed, aimed at extracting NMR data from the bore hole walls, including U.S. Pat. Nos. 4,350,955; 4,629,986; 4,717,877; 4,717,878; 4,717,876; 5,212,447; 5,280,243; and xe2x80x9cRemote xe2x80x98Inside Outxe2x80x99 NMRxe2x80x9d, J. Magn. Res., 41, p. 400, 1980; xe2x80x9cNovel NMR Apparatus for investigating an External Samplexe2x80x9d, Kleinberg et al., J. Magn. Res., 97, p. 466, 1992.
The apparatuses disclosed in the above documents are based on several permanent magnet configurations designed to create relatively homogeneous static magnetic fields in a region external to the apparatus itself RF coils are typically used in such apparatuses to excite the nuclei in the homogeneous region and, in turn, receive the created NMR signal. To create an external region of a homogeneous magnetic field, the magnetic configurations have to be carefully designed, to reconcile the fact that small deviations in structure may have a disastrous effect on magnetic field homogeneity. It turns out that such a region of a homogeneous magnetic field can be created only within a narrow radial distance around a fixed position relative to the magnet configuration, and that the characteristic magnetic field intensities created in this region are generally low. As a result, such apparatuses, although permitting NMR measurements, have only limited use as imaging probes for imaging extensive regions of bore-hole walls.
With respect to medical MRI-based applications, the potential of using an intra-cavity receiver coil has been investigated, and is disclosed, for example, in the following publications: Kandarpa et al., J. Vasc. and Interventional Radiology, 4, pp. 419-427, 1993; and U.S. Pat. No. 5,699,801. Different designs for catheter-based receiver coils are proposed for insertion into body cavities, such as arteries during interventional procedures. These coils, when located close to the region of interest, improve reception sensitivity, thus allowing high-resolution imaging of these regions. Notwithstanding the fact that this approach enables the resolution to be substantially improved, it still suffers from two major drawbacks: (1) the need for bulk external setup in order to create the static homogeneous magnetic field and to transmit the RF excitation signal; and (2) the need to maintain the orientation of the coil axis within certain limits relative to the external magnetic field, in order to ensure satisfactory image quality. Because of these two limitations, the concept of an intra-cavity receiver coil is only half-way towards designing a fully autonomous intra-cavity imaging probe.
U.S. Pat. No. 5,572,132 discloses a concept of combining the static magnetic field source with the RF coil in a self contained intra-cavity medical imaging probe. Here, several permanent magnet configurations are proposed for creating a homogeneous magnetic field region external to the imaging probe itself, a manner somewhat analogous to the concept upon which the bore-hole apparatuses are based. Also disclosed in this patent are several RF and gradient coil configurations that may be integrated in the imaging probe in order to allow autonomous imaging capabilities. The suggested configurations, nevertheless, suffer from the same problems discussed above with respect to the bore-hole apparatuses, namely: a fixed and narrow homogeneous region to which imaging is limited, and low magnetic field values characteristic of homogeneous magnetic field configurations.
There is accordingly a need in the art to improve MRI based techniques, by providing a fully autonomous intra-cavity MRI probe and an imaging method.
The present invention is based on the realization that rather than attempting to overcome problems of non-homogeneity of the magnetic field, this non-homogeneity may be used to the advantage of high-resolution imaging. The imaging probe according to the invention comprises all components necessary to allow magnetic resonance measurements and imaging of local surroundings of the probe, obviating the need for external magnetic field sources. The imaging method is based on the non-homogeneous static magnetic field created by permanent magnets and on a high sensitivity RF coil block, all located in the imaging probe itself. This makes the imaging probe an autonomous high-resolution magnetic resonance imaging device, capable of imaging the medium surrounding the probe.
There is thus provided according to one aspect of the present invention, a method for detecting NMR signals coming from a medium, the method comprising:
(i) producing a primary, substantially non-homogeneous, external magnetic field in the medium;
(ii) detecting the magnetic resonance signals from within at least one region of said primary, substantially non-homogeneous magnetic field.
The method enables the simultaneous detection of NMR signals originating from nuclei residing in the non-homogeneous primary magnetic field, and characterized by a substantially wide frequency range with respect to a mean frequency value. The term xe2x80x9csubstantially wide frequency rangexe2x80x9d used herein signifies the wide frequency range as compared to that utilized by conventional techniques, which is limited to 1% of the mean value. Thus, the wide frequency range is, generally, more than 1% of the mean value.
According to another aspect of the present invention, there is provided a probe for producing Nuclear Magnetic Resonance (NMR) signals coming from a medium surrounding the probe and detecting the produced signals, the probe comprising:
a magnetic field-forming assembly that produces a primary, substantially non-homogeneous, external magnetic field; and
a transceiver unit comprising at least one coil block capable of detecting magnetic resonance signals within at least one region of said primary magnetic field, said at least one region extending from the probe up to distances substantially of the probe dimensions.
The receiving coil block has sufficiently high sensitivity, namely such a sensitivity that enables the signal detection with sufficiently high signal-to-noise ratio from even a small volume cell of the medium located at a substantially large distance from the probe. For example, the following condition is indicative of the sufficiently high sensitivity: the NMR signal-to-noise ratio per volume cell (voxel) accumulated in a time frame of less than one minute (constituting the averaging time period) exceeding a value of five can be obtained, wherein the NMR signal part is originated from nuclei in the voxel of 0.1xc3x970.2xc3x971 mm in size located in the primary magnetic field at a distance up to the probe dimensions between the center of the voxel and the closest part of the probe. The noise part relates to the noise level of the coil block integrated over a frequency bandwidth equal to the bandwidth of the NMR signal originating from the same voxel.
Preferably, the coil block is composed of an RF coil, typically wound around a substantially toroidal core. The at least one region of the sufficiently high sensitivity is provided by forming the coil block with at least one narrow core gap, so that the gap plane is aligned substantially parallel to the direction of the primary magnetic field in the region of sufficiently high sensitivity. Several such spaced-apart core gaps may be provided so that more than one region of sufficiently high sensitivity can be created. The RF coil block may serve for both signal transmission (i.e., generates an RF magnetic field for exciting the nuclei), and for signal reception. Alternatively, the transceiver may comprise a separate element (e.g., coil block) for generating the transmission RF magnetic field.
Preferably, the field-forming assembly comprises two permanent magnets (which may be shaped as cylindrical rings) positioned in an axial, spaced-apart relationship on a common cylindrical ferromagnetic core, with their symmetry axes coinciding and defining the Z-axis, and a small inter-magnet gap remaining between the magnets. The magnets are magnetized along the X-axis perpendicular to the Z-axis and in opposite directions to each other, that is the N- and S-poles of one magnet face, respectively, the S- and N-poles of the other magnet. The assembly formed by the permanent magnets on the magnetic core creates the primary static magnetic field, which externally to the probe assembly and in the symmetry plane perpendicular to the Z-axis (hereinafter at times the xe2x80x9cX-Y-planexe2x80x9d) is directed substantially in the +/xe2x88x92Z direction with a maximum intensity obtained along the X-axis. The operation of the probe defines an imaging or measurement slice as the region where changes in the magnetic field component along the Z-axis are substantially small with respect to changes in the position along the Z-axis. By changing the size of the inter-magnet gap, the profile of the static magnetic field in the imaging slice can be varied. The RF coil block is preferably located in the gap between the magnets.
As for the RF coil block, when used for signal reception, the at least one region of the primary magnetic field from which the magnetic resonance signals are detected is located in proximity of the coil gap(s). In this region, the coil is substantially sensitive to variations in the transverse nuclear magnetization, i.e., in the X- or Y-component, depending on the RF-coil winding method. If the reception RF-coil block is used for transmission as well, the magnetic flux lines of the transmission magnetic field are substantially perpendicular to the Z-axis, and the transmission magnetic field intensity is highest in proximity of the coil gap(s). For both reception and transmission purposes, the at least one region of sufficient reception sensitivity and of maximum transmission intensity, respectively, can be visualized as at least one angular segment of the X-Y-plane, which is symmetrical with respect to the X-axis.
The field-forming assembly and the RF coil block may have the capability of revolving, together or separately, around the Z-axis. This revolution results in rotation of the angular segment(s), thereby scanning the imaging slice.
The device may also comprise a gradient coil to create variable magnetic field gradients in the lateral direction, i.e. perpendicularly to the radial direction and to the Z-axis (a so-called xe2x80x9cxcfx86-gradient coilxe2x80x9d).
The static magnetic field strength decreases with the increase of radial distance from the Z-axis, creating a substantially static magnetic field gradient in the radial direction. This can be utilized for creating a radial image dimension. When the probe is positioned at a fixed angle, i.e. without rotation about the Z-axis, the device can be used for obtaining pseudo-one-dimensional cross-sections of the surrounding medium residing substantially along the X-axis, i.e., along the radial passing through the RF-coil gap. An additional image dimension may be created by several techniques. As indicated above, one such technique utilizes a xcfx86-gradient oil that can be added to the probe configuration, as a means to achieve lateral separation (i.e. in the xcfx86 direction). Alternatively, lateral separation may be achieved by other methods, such as angular selective excitation or special signal processing.
Preferably, the probe is slowly rotated around the symmetry axis. A wide-band (non-selective) multiple-spin-echo excitation scheme may be used to acquire the magnetic resonance signal created by nuclei in wide overlapping angular sectors external to the probe. The signals acquired may then be averaged in order to improve the signal-to-noise ratio, and processed to create a two-dimensional image in polar coordinates (r, xcfx86).
The probe may be integrated into an intravascular catheter and used for imaging a series of 2-D cross-sections of blood vessel walls. The cross section images will extend appreciably into the vessel walls, providing high-resolution characterization of wall morphology, such as the structure and content of existing atherosclerotic lesions.
The imaging probe of the invention for intravascular medical use is included within a catheter that has preferably at least one hollow lumen to permit flow of blood therethrough. Such a hollow lumen may be achieved by the use of a hollow, e.g. tubular, magnetic core supporting the two magnetic field forming members (e.g., permanent magnets). A catheter probe in accordance with an embodiment of the invention comprises at least one inflatable balloon. When inflated, the balloon fixes the imaging probe to the vessel walls, minimizing relative motion during the time of image acquisition, while not obstructing the blood circulation.
The imaging probe may have a variety of different designs to meet particular applications, all being within the scope of the invention as defined herein. Variations may be in shape, cross section (tubular, cylindrical, rectangular, polygonal, etc.), proportions, size, material properties (mechanical, electromagnetic, etc.) and the like.
For example, the static field symmetric with respect to rotations about the Z-axis can be created. This can be achieved by magnetizing the xe2x80x9cupperxe2x80x9d magnet (i.e., one of the two magnets positioned on the Z-axis) radially outward and the xe2x80x9clowerxe2x80x9d magnet radially inward, leaving the ferromagnetic core unchanged. It can further obviate the need for rotating the magnetic structure.
Multiple high-intensity RF field segments or segments of substantially high sensitivity can be created, directed along a series of angles (for example: along 0, 45, 90, . . . , 315 degrees) covering the X-Y-plane. This will obviate the need for rotating the RF-coil block.
A xe2x80x9cstackxe2x80x9d of RF-coil blocks or of magnets/RF-coil blocks aligned along the Z-axis can be provided for the simultaneous imaging of multiple slices (X-Y-planes).
The preferred structure in terms of static magnetic field strength is the use of the original permanent magnets, having only solid cylindrical shapes (not rings) and no ferromagnetic core, or with a ferromagnetic core connecting two periphery regions of the magnets (not centered around the Z-axis). In this configuration, the RF-coil block is not necessarily a perfect toroid, but it is again in the inter-magnet, gap and the entire magnets-coil combination revolves, since both field patterns are not symmetric for rotations around the Z-axis.
All the above configurations can be made solid, namely without an internal lumen for blood flow. When such configurations are used for intravascular applications, blood flow can be allowed externally to the probe.
The above-described transceiver unit comprising a coil block having sufficiently high sensitivity for receiving magnetic resonance signals within at least one region of the primary magnetic field can be advantageously used in any MRI- or NMR-aimed device, irrespective of the homogeneity or non-homogeneity of the primary magnetic field, which can be created by an external static magnetic field source.
There is thus provided according to yet another aspect of the present invention, a transceiver unit for use in a probe for detecting NMR signals of a surrounding medium, the transceiver unit comprising at least one coil block capable of detecting magnetic resonance signals within at least one region of a primary external magnetic field, wherein said coil block comprises a coil wound on a substantially toroidal core having at least one core gap.
According to yet another aspect of the present invention, there is provided a device for NMR measurements or magnetic resonance imaging of a medium, the device comprising an imaging probe to be located in the vicinity of said medium and connected to a control station for generating transmission pulses, and for receiving, processing and displaying data generated by the probe, the probe comprising:
(a) a magnetic field forming assembly that produces a primary, substantially non homogeneous, external magnetic field in the medium; and
(b) a transceiver unit comprising at least one coil capable of detecting magnetic resonance signals from within at least one region of said primary, substantially non-homogeneous magnetic field, said at least one region extending from the probe up to distances substantially of the probe dimensions.
Imaging of human blood vessels is a preferred embodiment of the invention and the description bellow refers specifically thereto. It should, however, be undoubtedly clear that the following description of the preferred embodiments does not limit the present invention, but rather serves only to illustrate the invention. It is clear that by routine design modifications, which are within the reach of the artisan, probes in accordance with the invention for other applications may be designed.